This invention relates to a process for preparing catalysts useful in the production of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly it is directed to the preparation of catalysts suitable for producing maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or a mixture thereof.
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or mixtures thereof with molecular oxygen or oxygen-containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve combining a vanadium compound, a phosphorus compound, and if desired, promoter element compounds in a reducing medium under conditions which will provide vanadium in a valence state below +5 to form catalyst precursors capable of being converted to an oxide. The catalyst oxide precursor is then recovered and calcined to provide active catalytic material.
It has been taught in the art that various pentavalent and trivalent phosphorus compounds are satisfactory phosphorus components for use in the preparation of mixed vanadium phosphorus oxide catalysts. Orthophosphoric acid has been designated by some in the art as a preferred component.
U.S. Pat. No. 3,238,254 to Kerr lists the use of various phosphorus compounds such as metaphosphoric acid, triphosphoric acid, pyrophosphoric acid, orthophosphoric acid, phosphorus pentoxide, phosphorus oxyiodide, ethyl phosphate, methyl phosphate, amine phosphate, phosphorus pentachloride, phosphorus trichloride, phosphorus oxybromide, and the like, in the preparation of catalysts containing the mixde oxides of vanadium and phosphorus.
U.S. Pat. No. 3,474,041 to Kerr contains the above disclosure, and additionally discloses that vanadium phosphorus catalysts may be reactivated and stabilized by adding to the catalyst an organophosphorus compound.
U.S. Pat. Nos. 3,907,707 to Raffelson et al., 4,149,992 to Mount et al. and 4,179,404 to Barone disclose the preparation of vanadium phosphorus oxide catalysts using trivalent phosphorus compounds such as orthophosphorous acid, pyrophosphorous acid, metaphosphorous acid and hypophosphorous acid. Other phosphorus sources may include phosphorus trioxide and organic phosphites. A pentavalent phosphorus compound additionally could be utilized.
U.S. Pat. No. 4,043,943 discloses the preparation of the vanadium phosphorus oxide catalyst in a liquid organic medium, preferably anhydrous, wherein the vanadium compound is reduced and solvated by gaseous HCl followed by reaction with the phosphorus compound.
The preparation of oxidation catalysts containing the mixed oxides of vanadium and phosphorus is disclosed in copending U.S. Ser. No. 106,786, assigned to our common assignee, wherein a vanadium compound is at least partially solubilized in an organic liquid medium capable of reducing at least a portion of the vanadium to a +4 valence state, and unsolubilized vanadium having a particle size larger than about 0.1 mm diameter is removed from the medium before addition of a phosphorus-containing compound.
The preparation of such catalysts is disclosed in co-pending U.S. Ser. No. 146,971, assigned to our common assignee, wherein partial reduction of a pentavalent vanadium compound is effected in the presence of a phosphorus compound in an organic liquid medium capable of reducing the vanadium.
U.S. Pat. No. 4,013,586 to Dolan et al. discloses the preparation of vanadium phosphorus oxide catalysts using organo phosphonates as the source of phosphorus. A pentavalent phosphorus compound additionally could be utilized.